The present invention relates to an impact rotation tool including an impact force generation unit that converts the power of a drive source to impact force, which is pulsed torque, and uses the impact force to rotate a shaft, to which a tip tool is coupled.
An impact rotation tool decelerates the rotation output of a motor, which is one example of a drive source, with a deceleration mechanism, uses hydraulic pressure or the striking action of a hammer to convert the decelerated rotation output to pulsed impact torque, and performs a tightening task or a loosening task with the impact torque. Impact rotation tools are often used in construction sites and assembly factories (for example, refer to Japanese Laid-Open Utility Model No. 1-106169 and Japanese Laid-Open Patent Publication Nos. 1-106169, 8-267368, 2010-12587, and 11-267981).
When using an impact rotation tool, a fastener such as a bolt or screw, may be over-tightened by a large torque. On the other hand, when attempting to avoid such over-tightening, a fastener may be inefficiently tightened thus causing the fastener to be fixed with strength that is lower than necessary. Accordingly, Japanese Laid-Open Utility Model No. 1-106169 and Japanese Laid-Open Patent Publication No. 8-267368 each describe an impact rotation tool that measures torque with a strain gauge, a torque sensor, or the like, which is arranged on a shaft, so that a fastener can be tightened with a predetermined torque. When the torque indicated by the output value of the sensor reaches a predetermined torque such as a target torque, the motor is stopped.
In this case, the shaft on which the sensor is arranged is a drive portion. Thus, the sensor output value has a tendency of containing noise. This hinders accurate torque measurement. In particular, a relatively large noise is easily generated in an impact rotation tool when an impact force is applied to the shaft. As a result, in the sensor output, it is difficult to distinguish impact pulses from noise. As a result, when noise is erroneously detected as torque and the detected torque affected by noise reaches the predetermined torque, the motor may be controlled in a predetermined driving state and thereby be stopped. In such a case, the impact rotation tool stops before the tightening torque reaches the predetermined torque. To cope with this problem, Japanese Laid-Open Patent Publication No. 11-267981 describes an impact rotation tool that includes a filtering means such as low-pass filter to remove noise from the sensor output of a strain gauge, which is arranged on a drive shaft of a pulse wrench.
However, the filtering means of Japanese Laid-Open Patent Publication No. 11-267981 results in the impact rotation tool having a complicated structure. This increases the manufacturing cost of the impact rotation tool. It is difficult to set the cutoff frequency of the filtering means that distinguishes impact pulses from noise and removes only noise. For example, in an impact wrench, which is one type of an impact rotation tool, changes in the torque are more sudden than the pulse wrench of Japanese Laid-Open Patent Publication No. 11-267981. Thus, it is further difficult to distinguish impact pulses from noise.